Check for a tension pin located on the lower half. This would keep the mount from rotating during playing etc. Drive this out with adrift pin punch and the mount should unscrew. Replace the pin when done. The mount does not need to be screwed together tightly, or the pin holes may not line up.

I did this and replaced a normal length top half tilter ball with a 9" extension ball from a broken tilter mount. Now I have an 18" hex bd mount with a permanent 9" tilter on top.

Thanks! I'm a little concerned about that pin.

You can get pins at most hardware stores if it has problems. Just make sure you have the correct pin punch to drive it out. Do that part way and grab it with some plyers to finish pulling it out. If you are unsure, take the top to the store and someone should be able to get you what is needed.